1. Field of the Invention
The present invention relates to the repairing of a defective pixel in an electroluminescence (EL) panel.
2. Description of the Related Art
One known type of flat display panel is an EL display panel. As opposed to a liquid crystal display (LCD) panel, an EL display panel is self-emissive. For that reason, and because EL display panels are flat and capable of bright, clear displays, their use is expected to spread widely. In particular, because organic EL elements can be driven at lower voltages compared to inorganic EL elements, widespread use of organic EL elements in various displays is anticipated.
An organic EL display is configured by arranging, in a matrix, a large number of pixels each composed of an organic EL element. While either a passive or active method can be used to drive an organic EL element, use of an active matrix method is considered preferable, as is the case with LCDs. More specifically, in an active matrix method, a switching element is provided for each pixel, and display of each pixel is controlled by controlling the corresponding switching element. In comparison to a passive method in which each pixel is not provided with a discrete switching element, an active matrix method can create a screen display of higher definition, and an active matrix method is therefore preferred.
In an LCD, each pixel is provided with only one switching element (TFT) which is directly connected to a pixel electrode. In an organic EL panel, in contrast, two TFTs and a capacitor are employed for each pixel. FIG. 5 shows an example configuration of a pixel circuit using thin film transistors (TFT) in an organic EL panel. An organic EL panel is configured by arranging such pixels in a matrix.
A gate line GL extending in a row direction is connected to the gate of a first TFT 10, which is an n-channel thin film transistor selected by the gate line GL. The drain of the first TFT 10 is connected to a data line DL extending in a column direction. The source of the first TFT 10 is connected to a storage capacitor CS, which in turn is connected on the other terminal with a capacitor line SL serving as a low-voltage power source. An intermediate point in the connection between the source of the first TFT 10 and the storage capacitor CS is connected to the gate of a second TFT 40, which is a p-channel thin film transistor. The source of the second TFT 40 is connected to a power line VL, while the drain of the second TFT 40 is connected to an organic EL element EL. The other terminal of the organic EL element EL is connected to a cathode power source CV.
With this arrangement, when the gate line GL is at level H, the first TFT 10 is turned on. Data provided in the data line DL at that point is stored in the storage capacitor CS. A current in the second TFT 40 is controlled in accordance with the data (potential) stored in the storage capacitor CS. A current then flows in the organic EL element EL in accordance with the current in the second TFT 40, resulting in light emission.
When the first TFT 10 is turned on, a video signal associated with the pixel is supplied through the data line DL. As a result, the storage capacitor CS is charged in accordance with the video signal supplied through the data line DL. A corresponding current then flows in the second TFT 40, thereby executing brightness control of the organic EL element EL. In other words, display of gradation in each pixel is achieved by adjusting the gate potential of the second TFT 40 so as to control the current flowing in the organic EL element EL.
In an organic EL panel as described above, a defect in the first TFT 10 or the second TFT 40 provided for each pixel may occur during the panel manufacturing process. When a TFT is defective in a manner such that the TFT always disallows a current from flowing in an organic EL element, the corresponding pixel merely generates a dark point among many light-emitting points. As such a point is visually unnoticeable, it is not considered a problem. On the other hand, when a TFT is defective in a manner such that the TFT allows a current to continuously flow in an organic EL element, the corresponding pixel generates a light-emitting point. Even if it is only one pixel, one light-emitting point among surrounding dark or black pixels is noticeable to a viewer and is therefore regarded as a fault. Accordingly, a process of deactivating or xe2x80x9cdimming outxe2x80x9d a defective pixel that generates a light-emitting point is commonly performed.
Such a process is performed because an organic EL panel including a predetermined number of dark points is not in any way considered an inferior product. By executing a process of dimming out light-emitting points, manufacturing yield of the panels can be greatly enhanced.
The deactivation process can be performed by disconnecting the wiring connected to the defective pixel. As in an LCD, the wiring between the second TFT 40 and the power line or pixel electrode may be disconnected using a visible light YAG laser or the like. In this manner, a light-emitting point can be deactivated, thereby eliminating the fault from the display.
However, when a deactivation process using a visible light YAG laser is executed, there may result damages in the cathode and further effects on display by other pixels. More specifically, in an active matrix type organic EL panel, a pixel is created by disposing a TFT on a glass substrate, forming an ITO anode over the TFT, subsequently laminating organic layers such as a positive-hole transport layer, an organic emissive layer, and an electron transport layer, and then forming a metal cathode over the aforementioned components. As such, a portion of the organic layers and the cathode are positioned above the TFT. In particular, the cathode extends almost entirely over the surface of the panel as the common electrode.
Accordingly, when a any of the TFT wiring is disconnected using a visible light YAG laser, the laser beam penetrates the cathode and often causes ablation or other damage. The resulting configuration of the cathode is such that holes are created at locations where ablation occurred. The ablation may further cause quality in the anode to deteriorate, which may lead to undesirable influence on display by the surrounding pixels. The disconnection by a laser is effected by vaporizing and scattering the substances located where the laser beam is irradiated. Consequently, a side portion of the organic layer of the organic EL element may become directly exposed to the space above the cathode. Degradation of the organic layer due to penetration of moisture, oxygen, or the like in the exposed portion can result in further defective pixels.
Moreover, when wiring is disconnected by ablation, the once scattered wiring material (typically metal) may attach to other wiring components and generate a short circuit.
The present invention relates to a dim-out method used in an organic EL panel for effectively dimming out a defective pixel.
According to the present invention, a dim-out process can be performed by irradiating a laser beam on a semiconductor layer of the defective pixel. More specifically, in this process, crystal structure of the semiconductor layer is destructed at a micro level to increase resistance, there by executing electrical disconnection. In this manner, a favorable dim-out process that achieves dimming out of defective pixels basically without damaging other components can be performed.